Produced by the BBC for Christmas 1972, the only disappointment with this film may be the budget special FX towards the end (apparently produced at Pinewood studios), but use your imagination, and this isn't really a problem.
If you allow yourself to get into the atmosphere of this film the story, script, acting, energy, characters and atmosphere are wonderful, and this is rightly considered a classic of the horror genre. Also it's somewhat of a time capsule in more ways than one. If you don't like 'old' drama, you probably won't like this, but if you appreciate timeless classics you probably will.
You might also like to note that in 2015 BBC Radio 4 remade a version in 'binaural sound' for their Hallowe'en Fright Night experience - available currently to listen to. I'd watch the film first though!
Screen it alone in a darkened room, when everyone else is sleeping...